1. Field of the Invention
The present invention relates generally to an anti-lock brake control system for a motor vehicle. In more particular, the invention is concerned with the anti-lock brake control system which is capable of performing an anti-lock brake control while taking into consideration discriminatively the vibrations in the velocity or speed of the wheels of the motor vehicle which take place when the motor vehicle is running on a rough road of an irregular or wavy road surface on one hand and the vibration of the wheel speed referred to as the judder which is brought about due to variation in the torque transmitted between a driving system inclusive of an internal combustion engine (hereinafter also referred to simply as the engine) and the driving wheels of the vehicle on the other hand. Further, the invention is concerned with a method of detecting discriminatively whether vibration of the driving wheel of a motor vehicle is brought about by what is known as a judder phenomenon or by a rough-road condition.
2. Description of Related Art
In general, in the anti-lock brake control system for the motor vehicle, the trend of wheels being locked is detected on the basis of the result of comparison between the wheel speed (given, for example, by rotation number of the wheel per minute or rpm) and an estimated velocity or speed of the motor vehicle or on the basis of deceleration of the wheel or the like, wherein braking hydraulic pressure applied to the wheel is adjusted or regulated so that magnitude of skid of the wheel relative to the road surface is maintained at a value close to a region in which the friction between the wheel and the road surface assumes a peak value, with a view to shortening the stopping distance of the motor vehicle, ensuring stability of the vehicle body and enhancing the manipulatability or driving performance of the motor vehicle. By way of example, in the conventional anti-lock brake control system known heretofore, decision is made to the effect that the wheel of the motor vehicle tends to be locked when behavior of the wheel such as a slip thereof which represents a sink of the wheel speed relative to the estimated vehicle speed attains a predetermined threshold value, whereupon a control for lowering the braking hydraulic pressure applied to the wheel is carried out.
In this conjunction, it is noted that when a motor vehicle is running on a so-called rough road having a irregular or wavy road surface, the wheel speed is caused to vibrate due to the roughness of the road surface. In that case, the braking hydraulic pressure may be lowered as a result of the anti-lock brake control, because the trend of the wheel being locked may be decided on the basis of the wheel vibration possibly detected as the slip of the wheel on the basis of the acceleration/deceleration of the wheel, as mentioned above. However, it will readily be appreciated that in that case, the anti-lock brake control for lowering the braking hydraulic pressure is intrinsically unnecessary because the wheel vibration does not indicate the wheel-lock trend or tendency but it is brought about by the roughness of the road surface. Thus, when the anti-lock brake control system performs the braking hydraulic pressure lowering control in response to the detection of the wheel vibration brought about by the rough-road condition, there may arise such situation that the anti-skid braking force applied to the wheel becomes insufficient. Under the circumstances mentioned above, such measures are adopted that when the roughness of the road (i.e., bad road) on which the motor vehicle is running is detected, the criterion of the decision for validating the braking hydraulic pressure lowering control is made more severe or the criterion of the decision for increasing the braking hydraulic pressure is altered to be less severe, in an effort to suppress the anti-lock brake control in response to the wheel vibration occurring when the motor vehicle is running on a bad or rough road.
Next, let's consider the judder phenomenon. When the braking hydraulic pressure is increased or decreased steeply, magnitude of the torque applied to the wheel varies significantly. Upon occurrence of such variation in the torque applied to the driving wheel of the motor vehicle, variation naturally takes place in the transmission of torque between the driving wheel and the engine which are operatively connected or coupled together via a drive shaft. In that case, because of a large inertia of the engine, the drive shaft coupling together the engine and the driving wheel is subjected to torsion. In particular, when the motor vehicle is running on a road having a road surface of a small coefficient of friction such as a frozen road surface, the drive shaft is likely to be applied with such torsion, which incurs vibration of the wheel. This phenomenon is what is referred to as the judder. In this conjunction, when the phenomenon of judder is erroneously identified as the vibration due to the rough-road condition to thereby set more severe the criterions for validating the lowering of the braking hydraulic pressure, there may arise such situation that the braking hydraulic pressure is not lowered for the driving wheel which has actually a tendency of being locked. Consequently, magnitude of the slip of the wheel increases, involving degradation in the stability as well as in the steering performance of the motor vehicle. On the other hand, when the criterions for validating the control for increasing the braking hydraulic pressure are set less severe, as mentioned previously, the judder will unwantedly be promoted because of the torque varies in synchronism with the vibration, to uncomfortableness of a drive of the motor vehicle.
As the measures for coping with the problems mentioned above, there have heretofore been proposed various approaches. By way of example, in Japanese Unexamined Patent Application Publication No. 32222/1994 (JP-A-6-32222), such an anti-lock brake control for a motor vehicle is disclosed according to which decision is made to the effect that wheel vibration is taking place when acceleration and deceleration of predetermined magnitude occurs a predetermined number of times within a predetermined time, wherein the wheel vibration is decided as being brought about by the judder when the frictional coefficient of the road surface is low.
Additionally, there is disclosed in Japanese Unexamined Patent Application Publication No. 257347/1995 (JP-A-7-257347) such an anti-lock brake control according to which when rapid restoration of the wheel speed of the driving wheel is detected and when the wheel speed of the non-driving wheel is restored within a predetermined time in succession to the detection of the rapid restoration of the wheel speed of the driving wheel, the criteria for validating the lowering of the braking hydraulic pressure are set severe. On the other hand, unless the wheel speed of the non-driving wheel is restored within the predetermined time in succession to the detection of the rapid restoration of the rotation speed of the driving wheel, it is then decided that the wheel vibration is ascribable to the judder phenomenon, and thus the criteria for lowering the braking hydraulic pressure are modified to be more lenient.
As is apparent from the above description, in the case of the anti-lock brake control system disclosed in JP-A-6-32222, it is certainly possible to discriminate whether the vibration of the driving wheel is bought about by the rough-road condition or ascribable to the judder phenomenon. However, the discriminative decision of the judder is made on the basis of the estimated coefficient of friction of the road surface. Accordingly, difficulty is encountered to decide discriminatively whether the vibration of the driving wheel is due to the rough-road condition or the judder phenomenon, when it is difficult to estimate the coefficient of friction of the road surface. In reality, estimation of the coefficient of friction of the road surface is difficult in the early phase of the anti-lock brake control because deceleration of the motor vehicle is not stable during such early period.
In the case of the anti-lock brake control system disclosed in JP-A-7-257347, the vibrations of both the driving wheel and the non-driving wheel are detected, wherein when the rotation speeds of both the driving wheel and the non-driving wheel are vibrating, it is then decided that the vibration of the wheels is ascribable to the rough-road condition whereas when only the speed of the driving wheel is vibrating, decision is made that the vibration is due to the judder phenomenon. Accordingly, with this anti-lock brake control system, it is impossible to discriminatively discern whether the wheel vibration is due to the rough-road condition or it is brought about by the judder phenomenon in e.g. a four-wheel driven motor vehicle whose wheels are all implemented as the driving wheels, giving rise to a problem.